1. Field of the Invention
This invention deals with a thermoplastic elastomer composition having improved adhesion to polar materials, e.g. polar substrates, without requiring pre-treatment of such polar materials or the use of additional adhesives.
2. Description of Related Art
Thermoplastic elastomers are typically materials which exhibit properties between those of crystalline or glassy plastics and soft elastomers. To be considered thermoplastic they must soften upon heating, such that in the molten state they are capable of being shaped by plastic processing techniques, such as injection molding or extrusion.
Blends or alloys of plastic and rubber have become increasingly important in the production of high performance thermoplastic elastomers, particularly for the replacement of thermoset rubber in various applications. Among the more commercially relevant thermoplastic elastomers are those based on physical blends of polyolefins and rubbers.
Various blended products are described in U.S. Pat. Nos. 3,806,558, 3,835,201, 3,957,919, 4,130,535 and 4,311,628. These patent documents generally concern compositions which are comprised of polyolefin resins containing either uncured, partially cured, or fully cured polyolefin rubbers. According to these patent documents further improvements in physical properties such as tensile strength, elongation, and compression set are achieved when the rubber phase is well dispersed into small particles of fixed size. This occurs by curing the rubber in its dispersed state without curing the plastic phase so as to maintain its thermoplasticity.
Another type of thermoplastic elastomer which is frequently used is a thermo-elastomeric styrene based blockcopolymer, or its blend with a polyolefin.
These thermoplastic elastomer compositions are non-polar in nature. Consequently, ensuring that such materials adhere to a polar material presents a considerable challenge. The most common efforts focus on using substrate pre-treatments to improve adhesion or bonding. Industrially practiced pretreatment methods depend on the substrate and include solvent etching, sulfuric acid or chromic acid etching, sodium treatment, ozone treatment, flame treatment, UV irradiation, and plasma treatment. These procedures are costly, use hazardous materials, result in product degradation, and create environmental hazards.
Still other efforts include those described in U.S. Pat. No. 4,957,968, the complete disclosure of which is incorporated herein by reference, in which adhesive thermoplastic elastomer compositions comprised of blends of thermoplastic elastomer (TPE) containing about 15 to 40 wt. % of grafted maleic anhydride polypropylene are applied to substrates. The maleic anhydride (MAR) apparently functions as a means for introducing polarity into the formulation and to react with other reactable functional groups. In the case of bonding to a polyamide-based substrate, both such substrate and the TPE must be in the molten state to initiate the covalent bonds necessary for the adhesion. Adhering to a cold substrate using this so-called adhesive thermoplastic elastomer is generally unsuccessful. Little or no covalent bonds will form at the interface and the adhesion is therefore inadequate.
Another effort is detailed in the PCT International Patent publication WO 95/26380. According to this patent publication, adhesive thermoplastic elastomer compositions can be comprised of blends of thermoplastic elastomers with a reaction product of a functionalized polyolefin and a polyamide. These adhesive thermoplastic elastomers are said to provide improved surface properties and to adhere to engineering resins such as nylon (or polyamide). The improvement asserted over the U.S. Pat. No. 4,067,068 is that maleated polypropylene (PP) is grafted with low molecular weight polyamide to form an adhesion promoter (grafted nylon-MAH-PP). This grafted nylon-MAR-PP then is blended with the selected thermoplastic elastomer to yield the so-called adhesive product. In this case, the need for temperature-dependent covalent bonding across the interface between the composition and a polar material (engineering resin etc.) is eliminated and the polar segment of the dispersed adhesion promoter provides adhesion. Based on the Examples, the disclosed adhesive thermoplastic elastomer compositions would appear to be capable of bonding to Nylon 6.
However, the above-mentioned efforts suffer from some serious drawbacks. One deficiency is that blending the grafted nylon-MAH-PP with the selected thermoplastic elastomer results in a substantial increase in the overall hardness of the resulting thermoplastic elastomers which is, as can also be learned from the examples of the PCT-application, on the order of about 15 points (Shore A hardness). To maintain the starting hardness of the thermoplastic elastomer, common additives such as a block copolymer of styrene/conjugated diene/styrene are used. Compensating for the increase in hardness by starting from or adding amounts of a lower hardness thermoplastic elastomer results in a product exhibiting an inferior combination of physical properties.
Therefore a need remains for a composition which is capable of being adherent to a substrate, such as a polar substrate, without requiring pre-treatments while avoiding the disadvantages associated with the alternatives noted hereinabove.